faucet-sink-postgres

PostgreSQL sink connector for the faucet-stream ecosystem. Writes JSON records to a Postgres table over a pooled sqlx connection, batching them into multi-row INSERT statements for high throughput.

Reach for it whenever you want to land a faucet-stream source — a REST API, a CDC stream, a file, a queue — into Postgres with one declarative config and no glue code. Store records verbatim as a single jsonb column for schemaless ingestion, or map JSON keys straight onto typed table columns. With write_mode: upsert and a CDC source it becomes a live mirror; with delivery: exactly_once it commits records and a watermark in one transaction.

Feature highlights

• Two column-mapping modes — JSONB stores each record as a single jsonb value (schemaless ingestion, query later with JSON operators); AutoMap maps top-level JSON keys directly onto typed table columns discovered from the Postgres catalog.
• Multi-row INSERT batching — each page is written with one multi-row INSERT per chunk; JSONB mode uses unnest($1::jsonb[]), AutoMap binds per-column casts. Auto-splits to stay under Postgres' 65 535 bind-parameter ceiling.
• Write modes: upsert & delete — merge or remove rows by key via INSERT … ON CONFLICT (key) DO UPDATE (AutoMap + a UNIQUE/PRIMARY KEY on the key columns required). Last-write-wins de-dup within a batch.
• Exactly-once delivery — records and a monotonic commit token UPSERT into a _faucet_commit_token watermark table inside the same transaction; resume skips already-committed pages.
• Dead-letter queue — per-row partial writes route missing/null-key rows to a configured DLQ while good rows still commit.
• Connection pooling — one sqlx::PgPool built once in new() and reused for every batch; max_connections is configurable.
• Schema-qualified targets — optional schema scopes both column discovery and the INSERT target, so a same-named table in another schema can't pollute the AutoMap column set.
• Credential-safe logging — the Debug impl masks connection_url with ***.

Installation

# As a library:
cargo add faucet-sink-postgres
cargo add tokio --features full

# Or via the umbrella crate:
cargo add faucet-stream --features sink-postgres

# In the CLI (opt-in connector feature):
cargo install faucet-cli --features sink-postgres

Quick start

# pipeline.yaml — faucet run pipeline.yaml
version: 1
pipeline:
  source:
    type: rest
    config:
      base_url: https://api.example.com
      endpoint: /v1/users
  sink:
    type: postgres
    config:
      connection_url: postgres://writer:pass@localhost:5432/app
      table_name: users
      column_mapping: auto_map

faucet run pipeline.yaml

Configuration reference

Core

Batching

Write mode

Column mapping

column_mapping is the adjacently-tagged PostgresColumnMapping enum:

Examples

JSONB mode — store entire records in one column

Ideal for schemaless ingestion: store raw JSON and query it later with Postgres' JSONB operators.

CREATE TABLE raw_events (
    id         SERIAL PRIMARY KEY,
    data       JSONB NOT NULL,
    created_at TIMESTAMPTZ DEFAULT NOW()
);

sink:
  type: postgres
  config:
    connection_url: postgres://writer:s3cret@db.example.com:5432/analytics
    table_name: raw_events
    column_mapping:
      jsonb:
        column: data
    batch_size: 1000
    max_connections: 5

AutoMap mode — map JSON keys to typed columns

Discovers column names and types from the table schema and maps matching JSON keys. A field present only in some records is still written; rows missing a column bind SQL NULL.

CREATE TABLE events (
    user_id   TEXT,
    event     TEXT,
    timestamp TIMESTAMPTZ,
    amount    NUMERIC
);

sink:
  type: postgres
  config:
    connection_url: postgres://writer:s3cret@db.example.com:5432/analytics
    table_name: events
    column_mapping: auto_map
    batch_size: 1000
    max_connections: 10

CDC mirror — upsert with a delete marker

Pair a CDC source (run through the cdc_unwrap transform) with write_mode: upsert to keep a destination table in lock-step with the source.

pipeline:
  source:
    type: postgres-cdc
    config:
      connection_url: postgres://faucet:faucet@localhost:5432/appdb
      slot_name: faucet_slot
      publication_name: faucet_pub
  sink:
    type: postgres
    config:
      connection_url: postgres://writer:pass@localhost:5432/warehouse
      table_name: users
      column_mapping: auto_map
      write_mode: upsert
      key: [id]
      delete_marker:
        field: __op
        values: [d]

The destination must define the key as a constraint, e.g. CREATE TABLE users (id INT PRIMARY KEY, name TEXT, email TEXT).

High-throughput pool

sink:
  type: postgres
  config:
    connection_url: postgres://writer:pass@db-primary.internal:5432/warehouse
    table_name: metrics
    column_mapping: auto_map
    max_connections: 20
    batch_size: 1000

Streaming & batching

The sink re-chunks each incoming StreamPage so individual multi-row INSERT statements stay well under Postgres' per-statement bind-parameter limit.

• batch_size > 0 (default 1000) — slice the incoming page into batch_size-row chunks, one multi-row INSERT per chunk. 1000 is the recommended value — Postgres' multi-row INSERT sweet spot. AutoMap binds one parameter per column per row; the sink sub-splits each chunk further so rows × columns never exceeds Postgres' 65 535 bind-parameter ceiling, so a wide table never causes a rejected statement. JSONB mode binds a single jsonb[] array regardless of row count.
• batch_size = 0 — the "no batching" sentinel: the entire upstream page is forwarded in a single logical write. Use it when the source already emits Postgres-tuned page sizes. AutoMap still sub-splits internally to respect the 65 535-parameter ceiling.

batch_size is purely a chunk-size knob — connection pooling, identifier quoting, and JSONB vs AutoMap behaviour are unchanged.

Write modes (upsert / delete)

By default the sink appends every record. Set write_mode to upsert or delete to merge or remove rows by a key instead.

Requirements (validated in PostgresSink::new, before any connection is made):

• column_mapping: auto_map is required. Upsert/delete match on real columns, so the key columns must be table columns — not fields buried inside a JSONB blob.
• The key columns must carry a UNIQUE or PRIMARY KEY constraint, since upsert is implemented with INSERT … ON CONFLICT (key) DO UPDATE SET … (non-key columns set from EXCLUDED; if every column is a key column the clause degrades to DO NOTHING). Without the constraint Postgres rejects the ON CONFLICT target.

Semantics:

• Last-write-wins within a batch. When the same key appears multiple times in one write_batch call, the records are de-duplicated to a single effective action (the final one), so a single statement never hits the same ON CONFLICT target twice. A delete after an upsert (or vice-versa) for the same key resolves to whichever came last.
• write_mode: delete routes every record to a delete by key.
• A record missing a key column (or with a null key value) fails with a typed Sink error. When a dlq: block is configured the good rows are still written (upserts + deletes applied) and only the missing/null-key rows are routed to the DLQ per-row; without a DLQ the whole batch fails.

The cdc_unwrap transform pairs naturally with upsert — it normalizes a CDC envelope into a flat row plus a __op marker ("u"/"d") that the delete_marker matches. See the upsert cookbook.

Exactly-once delivery

PostgresSink implements Sink::supports_idempotent_writes (returns true) and the two companion hooks:

• write_batch_idempotent(records, scope, token) — writes records and UPSERTs the token into a _faucet_commit_token(scope TEXT, token TEXT) watermark table inside the same transaction, so both either commit together or neither does.
• last_committed_token(scope) — reads the current watermark so the pipeline skips already-committed pages on resume.

To use exactly-once delivery, set delivery: exactly_once and pair this sink with a CDC source (postgres-cdc, mysql-cdc, mongodb-cdc) plus a state: block. A DLQ is not permitted in exactly-once mode. All four requirements are validated at config-load time (faucet validate) before any run starts.

version: 1
pipeline:
  source:
    type: postgres-cdc
    config:
      connection_url: postgres://faucet:faucet@localhost:5432/appdb
      slot_name: faucet_slot
      publication_name: faucet_pub
  sink:
    type: postgres
    config:
      connection_url: postgres://writer:pass@localhost:5432/warehouse
      table_name: change_events
      column_mapping: auto_map
  state:
    type: file
    config:
      path: ./state
delivery: exactly_once

delivery: exactly_once and write_mode: upsert compose — the upsert and the commit-token UPSERT commit in the same transaction. See the exactly-once delivery cookbook.

Schema evolution

PostgresSink reports its live destination schema via current_schema() (read from pg_catalog, including attnotnull so nullability round-trips), so the pipeline-level schema: policy can detect drift between an incoming page's top-level shape and the real table. All five on_drift modes (warn / ignore / quarantine / fail / evolve) work against this sink.

Under on_drift: evolve, PostgresSink::evolve_schema() applies additive DDL in one connection:

• New columns → ALTER TABLE … ADD COLUMN IF NOT EXISTS (idempotent).
• Lossless widenings (e.g. integer → number) → ALTER COLUMN … TYPE — gated on allow_type_widening.
• Nullability relaxations (a previously NOT NULL column absent from the page) → ALTER COLUMN … DROP NOT NULL.

Incompatible changes (narrowing / type swaps) are never auto-applied — they are routed by on_incompatible (fail or quarantine). See the schema-drift cookbook.

Dead-letter queue

The sink overrides Sink::write_batch_partial, so when a dlq: block is configured the router gets per-row outcomes: good rows commit and only the failing rows (e.g. missing/null key columns under write_mode: upsert/delete) are wrapped in a DLQ envelope and routed to the DLQ sink — the batch is not aborted. Without a DLQ, a row failure fails the whole batch. See the DLQ cookbook.

Config loading & schema introspection

Load from YAML/JSON, environment variables, or a .env file via faucet_core::config:

use faucet_core::config::{load_json, load_env_file};
use faucet_sink_postgres::PostgresSinkConfig;

let config: PostgresSinkConfig = load_json("config.json")?;
let config: PostgresSinkConfig = load_env_file(".env", "PG_SINK")?;

# .env (prefix PG_SINK)
PG_SINK_CONNECTION_URL=postgres://writer:s3cret@db.example.com:5432/analytics
PG_SINK_TABLE_NAME=raw_events
PG_SINK_COLUMN_MAPPING='{"jsonb":{"column":"data"}}'
PG_SINK_BATCH_SIZE=1000
PG_SINK_MAX_CONNECTIONS=5

Inspect the full JSON Schema with:

faucet schema sink postgres

Library usage

use faucet_core::{Pipeline, Sink};
use faucet_sink_postgres::{PostgresColumnMapping, PostgresSink, PostgresSinkConfig};
use serde_json::json;

# async fn run() -> Result<(), Box<dyn std::error::Error>> {
let config = PostgresSinkConfig::new("postgres://writer:pass@localhost:5432/app", "events")
    .column_mapping(PostgresColumnMapping::AutoMap)
    .with_batch_size(1000)
    .max_connections(10);

let sink = PostgresSink::new(config).await?;

let records = vec![
    json!({"user_id": "u1", "event": "purchase", "amount": 29.99}),
    json!({"user_id": "u2", "event": "signup"}), // missing "amount" → NULL
];
let rows = sink.write_batch(&records).await?;
println!("wrote {rows} rows");
# Ok(())
# }

Drive it from a full pipeline:

use faucet_core::Pipeline;
use faucet_source_rest::{RestStream, RestStreamConfig};
use faucet_sink_postgres::{PostgresColumnMapping, PostgresSink, PostgresSinkConfig};

# async fn run() -> Result<(), Box<dyn std::error::Error>> {
let source = RestStream::new(RestStreamConfig::new("https://api.example.com", "/v1/users"));
let sink = PostgresSink::new(
    PostgresSinkConfig::new("postgres://writer:pass@localhost:5432/app", "users")
        .column_mapping(PostgresColumnMapping::AutoMap),
)
.await?;

let result = Pipeline::new(source, sink).run().await?;
println!("transferred {} records", result.records_written);
# Ok(())
# }

How it works

• A sqlx::PgPool is created once in PostgresSink::new() with the configured max_connections and reused for every batch.
• write_batch() slices records into batch_size chunks (or forwards the whole slice when batch_size = 0) and inserts each chunk with a single multi-row INSERT.
• JSONB mode inserts via INSERT INTO table (col) SELECT * FROM unnest($1::jsonb[]) — one bound array, no per-row parameters.
• AutoMap mode queries each column's name and underlying type (udt_name) from the catalog, scoped via to_regclass to exactly the relation the INSERT targets (the configured schema, else the search_path-resolved table). A multi-row INSERT INTO ... VALUES ($1::int4, $2::timestamptz), ... is built dynamically with a per-column cast; each value is bound as text so the destination column's input function parses it — numbers, booleans, timestamps, uuids, and json/jsonb columns all land in their native types. The column set is the union of record keys across the batch (in declared table order); a row missing a column binds SQL NULL.
• All identifiers (table + column names) are quoted with quote_ident() to prevent SQL injection.

Lineage dataset URI

postgres://<host>:<port>/<db>?table=<schema.table> (credentials stripped) — e.g. postgres://host:5432/app?table=public.orders.

Feature flags

This crate has no optional features of its own; enable it in the CLI/umbrella via the sink-postgres feature.

Troubleshooting / FAQ

See also

• Sinks reference — capability matrix across all connectors.
• Upsert cookbook — write modes and CDC mirroring.
• Exactly-once delivery — state, watermarks, supported source/sink set.
• Dead-letter queue cookbook — routing bad rows.
• faucet-source-postgres — the matching query source.
• faucet-source-postgres-cdc — the CDC source that pairs with upsert/exactly-once.

License

Licensed under either of Apache License, Version 2.0 or MIT license at your option.